Synopsis

The modelled concentration of ammonia in the atmosphere over the past years was found to be about 25% lower than the measured concentrations in the Dutch Monitoring Network. This is known as the ammonia gap. . Based on recent research carried out by RIVM in cooperation with PBL, WUR and ECN, it is concluded that a significant gap between modelled and measured ammonia concentrations no longer exists. With this finding a large uncertainty about the ammonia emissions and the compliance with the ammonia emission of the National Emission Ceiling Directive of the EU for 2010 has been taken away. The three reasons for the ammonia gap were studied, i.e. the uncertainties in a) the measurements of ammonia, b) the modelling of the ammonia concentrations with the PBL/RIVM OPS model and c) in the emissions of ammonia. The uncertainty in the ammonia concentration measurements was estimated to be approximately 7%. From recent literature and novel measurements it was concluded that the velocity with which ammonia is removed from the atmosphere, by vegetation and soil, was much smaller than was assumed in the OPS model. With this lower velocity the ammonia concentrations were calculated to be 15% higher. With this finding the ammonia gap was diminished to 10%. Emissions from crops especially during senescence are present which are not incorporated in the national emissions. This would account for about 4% of the national emissions. With these emissions the ammonia gap further reduces to about 5%. Given the uncertainties in the measurements and the modelling of the ammonia concentrations this gap cannot be considered as significant.